In recent years, in the automotive field, car bodies have been reduced in weight to reduce fuel consumption and cut CO2 emissions and have been improved in collision safety by use of high strength steel sheet for the car bodies and their parts. For assembly of car bodies and attachment of parts etc., spot welding is mainly being used. In spot welding of high strength steel sheets, the strength of the welded joint becomes an issue.
In a joint formed by superposing and spot welding steel sheets (below referred to as a “spot welded joint”), the tensile strength is an important characteristic. Normally, the mechanical characteristics of a spot welded joint are evaluated by the tensile shear strength (TSS) measured by applying a tensile load in the direction shearing the steel sheets and by the cross tensile strength (CTS) measured by applying a tensile load in a direction peeling apart the steel sheets. The methods of measuring the tensile shear strength and the cross tensile strength are defined in JIS Z 3136 and JIS Z 3137.
In tensile strength 270 to 600 MPa steel sheets, the CTS of the spot welded joint increases along with an increase in the steel sheet strength, so there is little chance of a problem arising relating to the joint strength. However, in tensile strength 750 MPa or more steel sheets, even if the tensile strength of the steel sheets increases, the CTS does not increase, but conversely decreases.
In general, in the case of high strength steel sheets, the concentration of stress in a weld increases due to the drop in the deformation ability. Further, due to the increase in the alloy elements, the weld is easily hardened and the fracture toughness of the weld falls. As a result, the CTS falls. For this reason, improvement of the CTS in the spot welded joint of 750 MPa or more high strength steel sheets is sought.
As the method of securing strength in a spot welded joint of high strength steel sheets, various methods have been proposed.
PLT 1 discloses a welding method which repeats idling of conduction and re-conduction while still applying a pressing force after the main conduction in spot welding, then holds that state so as to refine the structure inside the nugget and improve the breaking strength of the joint.
The “structure” in the nugget in PLT 1 means the final structures and not the solidified structures. In this method, the re-conduction eases the segregation, but the solidified structures are not refined. As a result, a sufficient fracture toughness cannot be obtained. Further, repetition of re-conduction and hold are necessary, so a long time is required for welding and the advantage of spot welding of the short welding time cannot be enjoyed.
PLT 2 discloses a method of running current for tempering after a certain time elapses from the end of the main conduction and annealing the spot welded joint (nugget part and heat affected zone) to lower the hardness.
In this method, the martensite transformation is substantially completed before the operation of running current for tempering, so a long cooling time becomes necessary and there was the problem that the advantage of spot welding of the short welding time cannot be enjoyed.
Further, as a method of securing strength in a spot welded joint of high strength steel sheets, a method of heating a weld by another heating means after welding has been proposed.
PLT 3 discloses a method of heating and tempering a weld at a high frequency after welding.
In this method, a separate step becomes required after welding and the work procedure becomes complicated. Further, a special apparatus becomes required for utilizing the high frequency.
PLT 4 discloses a method of forming a nugget by main welding, then running a current of the main welding current or more as post-conduction.
In this method, if making the post-conduction time longer, the nugget size becomes larger and, further, the nugget structure becomes the same as the structure of a usual weld.
PLT 5 discloses a method of spot tensile strength 440 MPa or more welding steel sheets comprising restricting the chemical composition of the steel sheets to C×P≤0.0025, P: 0.015% or less, and S: 0.01% or less, welding the sheets, then heat treating the weld by 300° C.×20 minutes or so to raise the strength in the peeling direction of the joint.
In this method, the steel sheets which can be used are limited. Further, a long time is required for welding, so the productivity is low.
PLT 6 discloses a spot welded joint of high strength steel sheets (tensile strength: 750 to 1850 MPa, carbon equivalent Ceq: 0.22 to 0.55 mass %) which defines the microstructure of the outside layer region of the nugget and the average particle size and number density of carbides in the microstructure.
PLT 6 only discloses a provision relating to carbides. With just this provision, it is not possible to obtain a highly reliable joint strength.
PLT 7 discloses a spot welding method giving mechanical vibration to a scheduled welding location to refine the structure of the weld when the scheduled welding location changes from a molten state to solidified structures.
In this method, there is little propagation of mechanical vibration to the melt zone. To obtain the above effect, the apparatus becomes large in size.